1. Field of the Invention
This application relates broadly to extractor mechanisms for firearms. More particularly, it concerns an improved form of extractor spring and the recess in the bolt that houses it for M4/M16 and other military automatic and semi-automatic firearms.
2. Description of the Prior Art
The ubiquitous M4/M16 firearms were designed with an extractor that is powered by a common helical compression spring that has one large loop that has an interference fit into a recess in the extractor. This spring sits well aft of the extractor pivot pin with the loose end of the sprint resting on a shelf above the firing pin. This spring is exposed to such high stresses during operation that in a very small number of cycles the spring is too weak to reliably extract all expended cartridge cases from the chamber of the weapon. This malfunction is known as a “failure to extract”. The short service life of the extractor spring has been the object of several redesign efforts. The U.S. military currently utilizes a spring with a rubber core added so that to the spring force is added the force required to deform the rubber with the resultant additive force being sufficient to extend the service life for an additional period of time. Unfortunately the effective spring rate of the rubber material is sensitive to the environmental temperatures so that the total force applied to the extractor varies considerably with ambient temperature. The rubber piece is also in intimate contact with the spring and grit from the environment can lodge into the rubber and act to abrade the spring wire as the weapon fires, thus prematurely weakening the spring. Lewis in U.S. Pat. No. 6,182,389 and Olson in U.S. Pat. No. 6,609,319 both described new bolt designs for the M4/M16 that solve the extractor spring failure problem by changing the extractor design and having two helical compression springs that fit beside the firing pin so that spring stresses can be reduced. The problem with both of these designs is that they require changing the complete bolt and barrel assemblies in the weapon in order to incorporate the alteration. Currently users of M4/M16 rifles attempt to replace the extractor springs at an interval which is considerably less than the actual average extractor spring life. This requires that the operators of these weapons keep track of the quantity of rounds fired so their weapon does not start experiencing “failure to extract” malfunctions before the extractor spring is replaced.
The present invention solves the extractor spring life issue by allowing a relatively simple modification to the bolt whereby a pocket is created that accepts a specially designed bridged extractor spring that works with the weapon's original, unmodified extractor. The pocket allows the bridged spring to straddle the firing pin so that there is sufficient spring length to maintain a proper force against the extractor without resulting in excessive torsional stress within the spring. A portion of the pocket beneath the bridge wire is also removed from the bolt so the bridge wire can fit between the extractor and the bolt. The bridge wire fits under the extractor and allows the standard extractor to properly depress the bridged spring coils as the weapon is cycled. The bridged extractor spring cannot be installed incorrectly as its design is symmetrical about the center line of the extractor and the bridge prevents inverting the spring.